Mycobacterium avium complex (MAC) and M. abscessus (MAB) are the two most common nontuberculous mycobacteria (NTM) that cause human disease, and the incidence of NTM infections has been increasing worldwide. Both the organisms are generally resistant to standard antimybacterial drugs and other antibiotics. Hence, there is an critical need for new antimicrobial agents to treat these infections. Capuramycins (CM) are a new class of nucleoside antibiotics that inhibit bacterial cell wall construction by blocking biosynthesis of peptidoglycan (PG). Because PG is unique to bacteria, antibiotics that inhibit its biosynthesis selectively target bacteria with less toxicity to the host. Even though PG is present in all bacteria, CM and analogues have a narrow spectrum of activity, with highest activity against Mycobacteria sp. We investigated several compounds in this class and identified SQ641 as the most active compound with excellent in vitro activity against M. tuberculosis and several NTM. Under this SBIR phase I research proposal, we intend to extend our in vitro studies with SQ641 to include more NTM strains and to investigate SQ641 activity against several different MAC and MAB strains in macrophages and appropriate animal models. These studies will determine whether SQ641 should advance to investigational new drug (IND)-directed preclinical studies in preparation for human clinical trials. PUBLIC HEALTH RELEVANCE: Mycobacterium avium complex (MAC) and M. abscessus (MAB) are the two most common nontuberculous mycobacteria (NTM) that cause human disease. Both are ubiquitous environmental organisms able to thrive in harsh conditions in soil or water. Both NTM can cause cavitary or nodular lung disease in individuals with preexisting lung conditions, or disseminated disease in HIV infected individuals with low blood CD4+ T-cell count (MAC) or infection of the soft tissues following trauma-inducing hospital procedures or cosmetic surgery (MAB). Infections with NTM are invariably acquired from environmental sources and are not contagious. Both MAC and MAB are generally resistant to standard anti-TB drugs and have variable susceptibility to second-line anti-TB drugs. Hence, the existing antimicrobial agents are inadequate to treat MAC and MAB infections and there is an immense need for new antimicrobial agents. Capuramycins (CM) are a new class of nucleoside antibiotics that inhibit bacterial cell wall by blocking biosynthesis of peptidoglycan (PG), a cell wall constituent unique to bacteria. Even though PG is present in all bacteria, CM are most effective against mycobacteria. We investigated several compounds in this antibiotic class and identified SQ641 as the most active, with excellent in vitro activity against a limited number of MAC and MAB strains. Under this SBIR phase I research proposal we intend to extend our in vitro studies with SQ641 to include more NTM strains and to investigate its activity against MAC and MAB in macrophages and appropriate animal models. These studies are essential to advance the drug for clinical evaluation in human NTM infections.